Cleavage of the complement proteins C3 and C5 by activation of the complement system yields the low molecular weight fragments C3a and C5a (anaphylatoxins). A prominent biological activity of C3a and C5a is contraction of isolated airway smooth muscle. In addition C5a is a potent bronchoconstrictor in vivo in the guinea pig. Our recent studies have demonstrated that intravascular complement activation with cobra venom factor markedly enhances a subsequent antigen-induced bronchoconstriction in IgE-sensitized guinea pigs. Thus, the purpose of the proposed research is two-fold: 1) to assess the direct action of C3a and C5a as bronchoconstrictors themselves and 2) to examine the indirect effect of complement activation and C3a/C5a generation in enhancing antigen-induced bronchoconstriction. C3a and C5a will be administered i.v. or by aerosol and the role of histamine and arachidonate metabolites in anaphylatoxin-induced bronchoconstriction will be assessed using pharmacological antagonists and radioimmunoassay measurements of released mediators. Granulocytes and platelets will be depleted using specific antisera in order to determine the dependence of anaphylatoxin-induced bronchoconstriction on these cells. The mechanism of enhancement of antigen-induced bronchoconstriction after complement system activation in IgE- sensitized guinea pigs will also be examined. Studies will determine if this enhanced allergic bronchoconstriction is a direct result of complement activation and C3a/C5a generation, a result of increased sensitivity of the airways to endogenous mediators, dependent on the presence of circulating granulocytes or platelets, or the result of increased release of endogenous bronchoconstrictors. Bronchoconstriction in vivo will be assessed in anesthetized guinea pigs using measurements of tracheal airflow and transpulmonary pressure for determination of pulmonary resistance and dynamic lung compliance. Antigen administration via the intravenous or respiratory route will be investigated in passively sensitized guinea pigs. These studies will provide important information regarding the mechanism of bronchoconstrictor action of C3a and C5a and thus help determine the extent of their potential contribution as bronchoconstrictors in obstructive airway disease. In addition, these studies will determine if complement system activation and C3a/C5a generation could be important determinants of the severity of an anaphylactic reaction.